The promising uses of organic electroluminescence (EL) devices as solid light-emission type, low-priced, large-area, full color display devices result in many developments. In general, a light emitting device is composed of a light emitting layer and a pair of counter electrodes between which the light emitting layer intervenes. When an electric field is applied to between both electrodes, electrons are injected from a cathode, and holes are injected from an anode. Further, the electrons and the positive holes are recombined with each other at the light emitting layer to produce an excited state. When this excited state returns to a ground state, energy is released as light, thereby obtaining luminescence.
The conventional light emitting devices have been high in driving voltage, and low in light emitting luminance and light emitting efficiency. Further, they have also been significantly deteriorated in characteristics, so that they have not come in practice. In recent years, a light emitting device in which thin films containing an organic compound having high quantum efficiency and emitting light at a low voltage of 10 V or less are laminated with each other has been reported (Applied Physics Letters, 51, 913 (1987)), and an interest is centered around it. In this device, a metal chelate complex is used as an electron transfer and light emitting material, and laminated with a hole transfer material (amine compound), thereby obtaining green luminescence of high luminance. The luminance reaches thousands of candelas per centimeter square at a voltage of 6–7 V in d.c. However, when considering a practical device, the development of a light emitting device having higher luminance and efficiency has been desired. Further, when considering the utilization thereof as a full color display or a light source, it is practically necessary to give the three primary colors or white. In the above-mentioned device, an aluminum complex of 8-quinolinol (Alq) is used as the light emitting material, so that the light emitting color is green. Accordingly, the development of light emitting materials giving other light emitting colors has been desired. Various light emitting materials emitting colors other than green have hitherto been developed. However, they have the problems of low light emitting luminance and efficiency, and poor durability.
Further, the conventional devices good in color purity and high in light emitting efficiency are ones in which charge transfer materials are doped with fluorescent dyes in slight amounts, and have the problems with regard to the reproducibility of device characteristics from the production point of view, and that the long-term use thereof causes a reduction in luminance and changes in color because of low durability of the dyes. As means for solving the problems, the development of materials having both the charge transfer function and the luminescent function has been desired. However, the materials that have hitherto been developed have the problem that the use of fluorescent dyes at high concentrations leads to a reduction in luminance by association.
On the other hand, organic light emitting devices realizing luminescence of high luminance are devices laminated with organic materials by vapor deposition. The fabrication of the devices by coating is preferred from the viewpoints of simplification of manufacturing processes, processability and enlargement of area. However, the devices fabricated by the conventional coating systems are inferior in light emitting luminance and light emitting efficiency to the devices fabricated by vapor deposition. It has been therefore a great problem to make it possible to emit light of high luminance at high efficiency.
Further, recently, various substances having fluorescence have been used for dyes for filters, color conversion filters, photographic material dyes, sensitizing dyes, dyes for dying pulp, laser dyes, fluorescent drugs for medical diagnoses and materials for light emitting devices have been used, and the demand has increased for them. However, there are not so many compounds high in color purity of blue and strong in fluorescent intensity, so that the development of new materials has been desired.